The present invention concerns an infinitely variable friction wheel transmission.
An infinitely variable friction wheel transmission of this type usually has input and output discs arranged coaxially on a common shaft, arranged together in pairs and whose inside surfaces are designed in toroidal shape, with friction wheels arranged between the pairs of input and output discs. These friction wheels are in frictional contact with both the input discs and the output discs, and transfer the torque to be transmitted from the input disc to the output disc by virtue of friction-force contact, whre the rotation speed of the friction wheels being the higher the larger is the distance between their point of contact with the input disc and the rotation axis. In contrast, the rotation speed of the output discs is higher the closer the point of contact between the friction wheel and the output disc is to the rotation axis. Accordingly, by swivelling the friction wheels, the rotation speed of the output discs can be infinitely variably adjusted as desired. For this purpose, the rotation axles of the friction wheels are in each case mounted on a carrier which can be controlled by means of a swivelling device.
Such an infinitely variable friction wheel transmission is described in detail in DE 197 54 725 by the present applicant. This transmission comprises two transmission units arranged coaxially with the input shaft, each transmission unit having an input disc and an output disc between which, in each case, are arranged two friction wheels, each friction wheel being attached on a carrier that can be swivelled. Both the input and the output discs are mounted on a torque shaft which can be displaced slightly in the axial direction relative to the input shaft. The input disc of one transmission unit is rotationally fixed with respect to the torque shaft, but is mounted on the latter so that it can slide axially on it. The input disc of the other transmission unit too is connected to the torque shaft in a rotationally fixed way by virtue of drive gearing. The two output discs of the two transmission units are arranged mirror-symmetrically and adjacent to one another in the transmission, and are arranged on a common bushing, so that a torque transmitted from one input disc to its associated output disc and a torque transmitted from the other input disc to its associated output disc, are transmitted from the two output discs in rotationally fixed connection with the bushing to a gear-wheel that meshes with a gearwheel of an output shaft. A roller-shaped pressing mechanism acts upon one of the input discs, which is mounted so as to be displaceable in the axial direction on the input shaft but is in a rotationally fixed connection with it.
In the known infinitely variable friction wheel transmissions, the transmission ratio is usually adjusted by moving the friction wheels tangentially with respect to the transmission axle, such that, however, swivel forces from the input and output discs act on the friction wheel arranged between them, since to transfer the torque these discs have to be pressed against the friction wheel. In conventional infinitely variable friction wheel transmissions the friction wheel in each transmission unit is arranged such that its swivel axis is positioned at the mid-point of the torus formed by the associated input and output discs. Thus, at the contact point of the friction wheel with its associated input and output discs, so-termed normal forces are produced when the transmission ratio is adjusted.
To avoid the possibility that the normal forces occurring during transmission ratio adjustment are unequal, the torque moment on the friction wheel resulting from this might produce an undesired change of the transmission ratio of the transmission, it has already been proposed in DE 198 26 057 by the present applicant to compensate for a possible difference in the normal forces by producing a control force, such that with the friction wheel held axially, this control force which leads to tilting of the friction wheel, can be applied to one of the two associated discs, while when one of the discs is axially fixed, the control force acts on the friction wheel.
It has also already been proposed to support the friction wheels in an infinitely variable friction wheel transmission by means of two connecting rods which oppose the occurring reaction forces. In this, the swivelling movement of the friction wheels is made possible by roller bearings; this design, however, has the disadvantage that the weight of the friction wheel transmission is higher and there is no coupling of the swivelling movements of two carriers arranged in a transmission unit for the friction wheels.
Further, an infinitely variable friction wheel transmission has been proposed, in which at the top and bottom ends of the carriers in any one transmission unit a traction element is provided, for example an endless cable, which passes around the corresponding ends of the carriers essentially in a circle and which, to produce synchronous swivelling movements of the carriers in opposite directions, is arranged in the form of a Figure-8, with a crossover point mid-way between the two carriers. This arrangement both takes up the reaction forces on the friction wheels and also synchronizes the swivel angle of the two associated carriers in each transmission unit.
Now, if the traction element is used in such manner that no differential angle is possible between the carriers, i.e., the play between the traction element and the carrier is very small, manufacturing tolerances can lead to forced slippage under the friction wheels. If the traction element is used such that there is large play between the traction element and the carrier, then the necessary coupling between the carriers will only exist when there is already a large differential angle between the two carriers.
The purpose of the present invention is to provide a coupling between the two carriers of any one transmission unit, which enables a certain, specified differential angle between the two carriers to be set but, at the same time, when a differential angle occurs, a restoring force on the corresponding carrier is produced.
According to the invention, then, it is provided that a compensating element is associated with each carrier in a transmission unit, which brings about effective coupling of the two carriers of the transmission unit such that when a differential angle occurs between the two carriers, a restoring force is exerted on the carrier. The compensating elements are advantageously inserted as connection elements in the traction element.
In an example of a preferred embodiment, the compensating element comprises a support disc in contact with a spring surrounded by a sleeve, which is fitted so that it can move within a bushing against the force of another spring, while the other section of the traction element is attached to the center of the support disc.
The provision of a compensating element for each traction element provides the advantage that when a differential angle occurs, a restoring force is exerted on both carriers, whose effect is to reduce the differential angle. At the same time, tolerances in the mounting for the traction element and in the traction element itself can be compensated by the gradual rise of the restoring force, without leading to forced slippage at the contact points.
The manufacturing tolerances of the components, i.e., the holder for the traction element and the traction element itself, can be correspondingly greater. The behavior of the infinitely variable friction wheel transmission can be affected by choosing different characteristics for the springs used.
At the same time, the stability of the friction wheel transmission is increased by this type of coupling; dampers provided in the compensating element can damp any additional oscillations that occur.